It depends. I wouldn't say that a metronome is helpful for everyone. After all nobody had metronomes anywhere in the world before the device was invented by Winkel in 1814. See Dietrich Nikolaus Winkel - yet there were many fine musicians before the invention of the metronome.

And there are many very musical cultures today, amongst the most rhythmical by reputation of any, e.g. Flamenco music to name one. They rarely use the metronome - Flamenco metronomes are mainly for people who want to learn as adults, from outside the culture. Another example would be Scottish fiddle music, where I live. Though a few traditional music players use metronomes, I think it is rare.

If you have a good sense of rhythm and play musically and well, depending on your musical culture also, you may not need it at all.

However it can also be very helpful for some people.

Some types of music require a very steady "metronomic" beat. So - that's the usual use for it, as a way to make sure your beat is steady.

However if you use it that way - it's worth knowing that it is natural for tempo to vary continually. We don't walk like robots. In the same way you don't play music like a robot. It would sound unmusical, for most types of music. Sibelius and such like programs have special modes that "humanize" the timings and rhythms. And as programmer myself of Tune Smithy - fractal music generator and microtonal composition tool which is used for musical fractals and algo comp - I added many features to help users make music with non metronomic rhythms.

If you listen carefully to someone who you admire, as a musician with a fine sense of rhythm, and even when they play what most people would call a "rock steady tempo", you are bound to hear continual variations in the tempo. For instance it might be a little faster for four measures, then slower for four. Often music alternates a little faster and then a little slower on alternate measures. Even dance music is like this, especially traditional dance. That's why many players of traditional music can't get on with a metronome at all - because it doesn't let you do a lively "dance rhythm" because it forces all the measures to be the same tempo, and all the beats within the measure too.

Note how almost every note played by Jaqueline du Pre in this next recording is different in timing with expressive rubato. Yet anyone would say she has an exquisite sense of timing and rhythm :).

In most musically expressive playing, the tempo varies continually, for instance here is a tempo plot of the Moonlight Sonata as played by Maurizio Pollini

That's true of all types of music. That is, unless the musicians use a click track - a click played over headphones. If they do, you can recognize this instantly, as a flat line, or almost flat line, in the tempo plot. For tempo plots of music to compare, see Revisiting the click track

You can also generate your own click plots or look up previously generated ones from that site (though the service seems not to be available right now - was available a week or two ago).

So - unless you do want to sound like one of those click track pieces - which is the goal in some styles of music indeed -  then you probably won't use the metronome or a click track for performances.

That depends on the style again. Some musicians have as their aim to sound like a click track - so in that case, yes you probably need a lot of training with a metronome to achieve that effect.

To others, this may sound boring, like a clock. Why would anyone want to learn to sound like a clock? The thing is that once you have heard three beats, you know exactly when the fourth one will happen. After a dozen or so you are in no doubt at all, you know to the millisecond or so, when every new beat will be.

Still, that doesn't mean that the metronome is useless. Because - it's actually much more versatile than you might think. It's like an instrument in its own right. You need to know how to use it to get the full potential from a metronome.

What a metronome can do is to help you to hone your sense of time. So you can hear fine details of rhythm to the millisecond, that you might not have heard before. And to perform with fine nuances of timing. Again some musicians can do this naturally so don't really need it. But for others, that's what it can help with. Also help with your sense of rhythm and tempo.

The key here is that you don't think of yourself as becoming a slave to the metronome. Rather the metronome is helping you to uncover your inner sense of tempo and rhythm. So you may do exercises such as set the metronome to go silent for several measures at a time (if it has that capability) and then come back in again and see if you are still in time.

This is Bounce Metronome set up to go silent briefly for that exercise:

Or play with the metronome fading to silence every four measures

Or  set it to play its ticks at longer and longer intervals, double, four times and so on, while you play at a steady tick and again aim is to be exactly in time with each of its ticks, spaced further and further apart.

These are two of the exercises from Mac Santiago's book "Beyond the Metronome".

http://inchronicity.com/

You don't need to be afraid that practicing in this way will make you metronomic. Far from it, it has the opposite effect. It's like an artist who demonstrates his or her fine control of a pen or brush by effortlessly drawing a perfect circle or line with one movement. It's just technique.

So, that's one way you can practice. And - if you practice like this, you do need to notice one thing. When you play with a metronome, most musicians instinctively play off the beat. Either just before, or just after, or randomly sometimes before and after. The reason is that they need to hear the metronome - and you hear it most clearly if you play off the beat. Play exactly on the beat and you mask its sound, it may even seem to skip a beat. But - if you want to use it for precision rhythm and tempo practice, that's what you have to do, to play exactly in time, so that your sound and the metronome's tick merge. And then also to be able to drift about in the beat, sometimes before, sometimes after - but in a controlled way. You play before because that's the exercise, not because you are trying to hit the beat and miss. And learn to hear that distinctive merge sound. Even if you play quite a loud instrument, still, you can learn to hear this merge, of yourself and the metronome playing together. It may feel as if you are actually playing the metronome :). On every tick.

There are many other ideas to use in your metronome practice.

See also Andrew Lewis's

Rhythm: What It Is And How to Improve Your Sense of It: Andrew C. Lewis

You can also check out my

available as a kindle booklet Vanishing Metronome Clicks, for Timing Sensitivity: And other Metronome Techniques - Many Ways to Use a Metronome

and also available to read for free online. The Vanishing Metronome Click - Burying the Click

I'm the author of Bounce Metronome, and there are many other videos to try out in thevideo resources section of the website. The program itself runs on Windows, Linux, Mac OSX (beta but pretty much finished), but not on iPad or Android. It's commercial software, but comes with a free taster metronome which can do the usual 2/4, 3/4, 4/4, 6/8, 9/8, 12/8 with up to four subdivisions, with many features. For some musicians that may be all you need

If you want to try the more advanced capabilities then it is paid for but with generous discounts if you need them. And a 30 day trial which you can renew as often as you want by asking for a new trial if you need one.

You can get it here: Check Out the Astonishing Bounce Metronome Pro

And this is an article I did about it: Metronomes - Do You Need Them? And A Metronome Using Conducting Techniques For Visual Precision

In that article I talk a lot about ways of practicing to help with a sense of rhythm, that don't involve using a metronome, towards the end of the article. Which might be worth reading though it is rather getting away from the topic of this answer.

BOOKS, AND ONLINE RESOURCES.

First the ones already mentioned, with numerous fun exercises to try.

Then, you can read Frederick Franz's Metronome Technique online for free, try especially his Chapter 2.His history section is an interesting read too.

Many other resources on my Metronome Links - Bounce Metronome page